Add a 'phase' enumeration field to the FeatureCompatibilityVersionDocument. This field will only be valid to be specified on shards. If the 'phase' is specified on a non-sharded cluster, the setFCV command will fail.

The phase will be one of 'start' (phase-1) and 'complete' (phase-2). The shard must verify that both phases of the same setFCV sequence have the same 'timestamp'.